"The proposed sale will increase the defensive capabilities of the Turkey military to guard against hostile aggression and shield NATO allies who might train and operate within Turkey's borders," a statement said.

Ankara a year ago announced a deal to purchase rival S-400 missiles from Russia, drawing a rebuke from its allies in NATO -- a group originally formed as a bulwark against the Soviet Union.

A purchase from Russia "would have serious ramifications for US ability to do business with Turkey across the defense trade spectrum," the official said.

"It is important that NATO countries procure military equipment that is interoperable with NATO systems. A Russian system would not meet that standard," he said.

The announcement comes hours after President Donald Trump said that the United States had defeated the Islamic State group in neighboring Syria -- leading to the withdrawal of US troops, who had effectively protected Turkey's campaign against Kurdish fighters.

There was no immediate indication that the two announcements were linked, although Trump has made clear that arms sales are a top priority for him.

Turkish President Recep Tayyip Erdogan on Monday vowed to "remove" the Kurdish fighters if necessary, as he said he had spoken by telephone with Trump and agreed on cooperation in Syria.